Electrodeless fluorescent lamps are disclosed in U.S. Pat. No. 3,500,118 issued Mar. 10, 1970 to Anderson; U.S. Pat. No. 3,987,334 issued Oct. 19, 1976 to Anderson; and Anderson, Illuminating Engineering, April 1969, pages 236-244. An electrodeless, inductively-coupled lamp, as disclosed in these references, includes a low pressure mercury/buffer gas discharge in a discharge tube which forms a continuous, closed electrical path. The path of the discharge tube goes through the center of one or more toroidal ferrite cores such that the discharge tube becomes the secondary of a transformer. Power is coupled to the discharge by applying a sinusoidal voltage to a few turns of wire wound around the toroidal core that encircles the discharge tube. A current through the primary winding creates a time-varying magnetic flux which induces along the discharge tube a voltage that maintains the discharge. The inner surface of the discharge tube is coated with a phosphor which emits visible light when irradiated by photons emitted by the excited mercury atoms. The lamp parameters described by Anderson produce a lamp which has a high core loss and is therefore extremely inefficient. In addition, the Anderson lamp is impractically heavy because of the ferrite material used in the transformer core.
An electrodeless lamp assembly having high efficiency is disclosed in U.S. patent application Ser. No. 08/624,043, filed Mar. 27, 1996 (now U.S. Pat. No. 5,834,905). The disclosed lamp assembly comprises an electrodeless lamp including a closed-loop, tubular lamp envelope enclosing mercury vapor and a buffer gas at a pressure less than about 0.5 torr, a transformer core disposed around the lamp envelope, an input winding disposed on the transformer core and a radio frequency power source coupled to the input winding. The radio frequency power source typically has a frequency in a range of about 100 kHz to about 400 kHz. The radio frequency source supplies sufficient radio frequency energy to the mercury vapor and the buffer gas to produce in the lamp envelope a discharge having a discharge current equal to or greater than about 2 amperes. The disclosed lamp assembly achieves relatively high lumen output, high efficacy and high axial lumen density simultaneously, thus making it an attractive alternative to conventional VHO fluorescent lamps and high intensity, high pressure discharge lamps.
Another type of electrodeless lamp is disclosed in U.S. Pat. No. 4,298,828 issued Nov. 3, 1981 to Justice et al. A globe-shaped lamp, wherein the discharge path is irregular in shape and is confined to an approximately spherical lamp envelope, is disclosed. A transformer core is located within the lamp envelope.
Yet another type of electrodeless lamp is disclosed in U.S. Pat. No. 5,239,238 issued Aug. 24, 1993 to Bergervoet et al. A transformer core is positioned in a reentrant cavity of a generally globe-shaped electrodeless lamp envelope.
The high wall temperatures of the lamp envelopes in the aboved-scribed lamps necessitates the use of mercury amalgams to ensure near optimum mercury vapor pressure during typical operation. Amalgams also have the advantage of substantially increasing the useful temperature range of the lamps. However, under some conditions, the amalgam temperature can drop below the optimum temperature range. In this case, output lumens and efficacy drop, and lamp color can shift due to the drop in mercury vapor pressure. These undesirable changes can occur in globe-shaped lamps, which do not have an integral ballast to provide amalgam heating, and also in tubular lamps. Temperatures below optimum can occur when lamp power is reduced during dimming and in low ambient temperatures, and also when the lamp is operated outside a fixture.
In tubular electrodeless lamps, the most practical location for the amalgam is in the exhaust or dummy tubulation. With lamps of typical loading operating in an indoor enclosed fixture, the amalgam temperature reaches about 85.degree. C. to 95.degree. C., well within the temperature range which gives lumens greater than 90% of peak. However for outdoor use, it is desirable to maintain high lumen output down to minus 20.degree. C. or lower. Under these conditions, lumen output can drop far below peak. Also, in open air at normal room temperature of 25.degree. C., the amalgam drops to below the temperature range that gives a lumen output greater than 90% of peak for common amalgam systems based on bismuth, tin and lead or bismuth and indium.
Accordingly, it is desirable to provide electrodeless lamp configurations and methods of operating electrodeless lamps which provide high lumen output over a wide range of operating temperatures.